Turbidity is an index to token the concentration of suspended particles and colloid in water, which can reflect the suspended particles content in water. The contents of bacteria, coliform, virus, ferro-manganese and so on in water can be reduced by reducing the turbidity of water, thus the turbidity of water is an important water quality index for water treatment. Thus, according to GB 5749-2006 Standards for Drinking Water Quality, turbidity is classed as a conventional index of water quality and its limiting value is 1 NTU.
Measuring technique of turbidity has a long history of development—from candle method to photometer and then to turbidimeter. At present, there are two measuring methods of international standard as follows: one is the “USEPA 180.1” and the other one is the “ISO7027”. The former is achieved by using a tungsten lamp as light source with a spectrum in a range of 400 nm to 600 nm, and it has poor stability and narrow range of testing; and the latter is achieved by using a LED as light source with a wavelength of 860 nm, and it has good testing stability but has low sensitivity for the water sample with low turbidity and especially for the one with turbidity less than 1 NTU.